In chemistry, a photocatalyst generally refers to a catalyst substance capable of facilitating a chemical reaction by receiving a light and a photocatalyst reaction generally refers to a catalytic reaction caused by an absorption of light. In the technical field of the conventional photocatalyst or photocatalysis, the frequently-used photocatalyst materials include such as a titanium dioxide (TiO2), an zinc oxide (ZnO), a tin dioxide (SnO2), a cadmium oxide (CdO), a chromium oxide (CrO2), a cadmium sulfide (CdS) and an zinc sulfide (ZnS) etc. The titanium dioxide is the most widely-used one among these materials to act as a photocatalyst medium, because of its good chemical stability, non-toxic feature and cheap price. In particular, the nanolized titanium dioxide is able to provide better ability of oxidation, specific surface and activity of photocatalysis.
In the state of the art, the traditional way to utilize the photocatalyst is often to coat the photocatalyst in the powder state onto a surface of the substrate as a thin layer of photocatalyst by using a chemical deposition scheme. The most frequently-used chemical deposition scheme includes an atomic layer deposition (ALD) scheme, a chemical vapor deposition (CVD) scheme, a plasma-enhanced chemical vapor deposition (PECVD) scheme etc., in which the ALD scheme is widely applicable to the technical field requiring growing a thin film, such as a ultra thin dielectric layer, a diffusion barrier layer, a seed layer, a sidewall, a sidewall oxide or a sidewall spacer etc., in such as a semiconductor component.
The ALD scheme is a well-known deposition technique and it employs a precursor material which can react with or chemisorb on a surface in process to build up successively deposited layers, each of which layers being characterized with thickness about only one atomic layer. Subject to properly selected process conditions, the chemisorption reaction has a self-limiting characteristic, meaning that the amount of precursor material deposited in every reaction cycle is constant and the precursor material is restricted to growing on the surface, and therefore the film thickness can be easily and precisely controlled by the number of the applied growth cycles.
Conventionally, a batch of ALD scheme usually consists of multiple ALD reaction cycles, each of which ALD reaction cycles involves consequently performing steps of introducing a first gaseous precursor pulse to a surface in process, pulsing an inert gas to purge or evacuate the excess gaseous precursor after the surface is saturated with an atomic layer of the first gaseous precursor, pulsing a second gaseous precursor and purging by an inert gas pulse or evacuating. A single ALD reaction cycle is continuously repeated until a target thickness for the deposited atomic layer on the surface in process is achieved.
It is to be noticed that in the technical field of the conventional photocatalyst, it is used to just form a single first layer of photocatalyst on the substrate. Actually, it is worth a try to further form an additional layer of photocatalyst on the first layer, which photocatalyst is a different type of photocatalyst to that of forming the first layer. It might be possible to enhance the entire reaction efficient by such a dual layer structure of photocatalyst.
There is a need to solve the above deficiencies/issues.